Because polished fused silica disks—traditionally used to make mirrors for imaging systems—have a relatively high CTE, mirrors made from them may deform, when subjected to heat generated during EUV illumination. Likewise, silicon wafers that have been considered for use as EUV mirror substrates may have an unacceptably large CTE. For that reason, both fused silica and silicon may not be suitable for making mirrors used in EUV lithography systems. When transmitting light onto such optical components, even slight warpage of the mirror's substrate, which can result from expansion upon heating, can cause a beam to be reflected from the mirror in an unintended direction.
Certain low CTE materials may be used instead to form mirror blanks. Although such materials' thermal and mechanical properties may make them more suitable than fused silica or silicon for making mirrors for EUV lithography, it may be difficult to give them the required high quality surface finish. To a certain extent, advanced polishing processes can reduce surface roughness, and adding a reflective multi-layer coating to the mirror blank may temper the impact of an uneven surface topology. Nonetheless, these material's irregular surface characteristics can cause light scatter in EUV imaging systems and remain a serious issue for EUV lithography.
Further advancements in polishing processes may not provide a satisfactory solution to this problem for the following reason. Certain low CTE mirror blank materials (e.g., those sold under the tradename ZERODUR® by Schott Glass Technologies) are composed primarily of silicates, e.g., orthosilicates, pyrosilicates, and metasilicates. The size of the crystallites, which make up these materials' quasi-random matrix, may impose upon them an inherent surface roughness that must exceed the 0.1 nanometer root mean square (“RMS”) roughness that EUV lithography lens designers desire. Because advancements in polishing techniques cannot change these crystallites' size, they can reduce the surface roughness of mirror blanks made from these materials to a limited extent only. Mirror blanks made from alloys are affected by grain boundaries, which also may limit how effectively advanced polishing processes can reduce surface roughness.
Accordingly, there is a need for an improved method for making a mirror for photolithography that includes a low CTE substrate. There is a need for such a method that reduces the surface roughness of that substrate. The present invention provides such a method for making such a mirror.